Need help on POW 12: The Big Knight Switch

The problem is that you have a 3x3 checkboard and you have 2 black knights on 2 corners and 2 white knights on the other corners. The goal is to switch them around in the least amount of moves. The white knights are at the corners where the black knights were and vice-versa. Help me with this please. I'm not a genius at chess. The answer is 16 moves, but how do you get 16 moves?